Browsing by Author "Robert J Nemanich, Committee Chair"

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    In Situ, Real-Time Characterization of Silicide Nanostructure Coarsening Dynamics by Photo-Electron Emission Microscopy
    (2007-05-30) Zeman, Matthew Casimir; Robert J Nemanich, Committee Chair
    Photo-electron emission microscopy (PEEM) was used to observe the growth and coarsening dynamics of transition metal (TM) silicide and rare earth (RE) silicide nanostructures on silicon surfaces in real-time with high lateral resolution during in situ annealing. Evidence was obtained indicating that the coarsening of the silicide islands is strongly influenced by local variations in the size, shape, and number of nanostructures on the surface. The titanium, hafnium, and zirconium silicide nanostructures were observed to grow via Ostwald ripening and attractive migration and coalescence (AMC) at temperatures as high as ˜1200°C. Ostwald ripening is a classic coarsening process in which larger nanostructures grow at the expense of smaller surrounding structures as per the Gibbs-Thompson relation. Attractive migration and coalescence is a newly discovered coarsening pathway where nearby islands are observed to migrate attractively towards each other and subsequently coalesce in response to local adatom concentration variations on the surface. A shape distortion of the normally compact and rounded TM silicide islands has been observed during these coarsening processes. The shape distortion suggests that the nanostructures are exchanging mass with each other via diffusion limited processes and these observations support the AMC model. The dysprosium and erbium silicide nanostructures exhibit a distinct faceted morphology and primarily coarsen via Ostwald ripening. The RE silicides form highly elongated nanowires and compact rectangular nanostructures on Si(001) and triangular or hexagonal structures on Si(111). Although the seemingly one-dimensional growth of the metallic nanowires is potentially promising for future microelectronics applications, we show that the wires are metastable structures which decay in favor of the larger rectangular islands at high temperatures. Additionally, the rectangular shape and faceted morphology of the RE silicide nanostructures greatly influences their coarsening dynamics. A separate PEEM study explored the thermal stability of thin films of TM oxides (TiO2, ZrO2, HfO2) grown on ultra-thin SiO2 buffer layers on Si(001) surfaces. The decomposition of the TM oxides was observed in the PEEM during annealing at ˜870°C, ˜900°C, and ˜1000°C for the TiO2, ZrO2, and HfO2 films, respectively. The degradation of the oxide films is attributed to a two-step reaction process which is initiated at defects in the TM oxide⁄SiO2⁄Si stack.
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    Spectroscopic Study of the Interface Chemical and Electronic Properties of High-kappa Gate Stacks
    (2005-08-11) Fulton, Charles Clifton; Jon-Paul Maria, Committee Member; Angus I Kingon, Committee Member; Robert J Nemanich, Committee Chair; Gerald Lucovsky, Committee Member
    X-ray and ultraviolet photoemission spectroscopy has been combined with in-situ deposition to study the interface chemistry and electronic structure of potential high-κ gate stack materials. In addressing the interface stability of ZrO2 with respect to a Si substrate three issues are considered: 1) the development of the band offsets and electronic structure during the low temperature (T<300°C) growth processes, 2) variations in the band structure as effected by process conditions and annealing (T<700°C) and 3) the interface stability of Zr oxide films at high temperatures (T>700°C). To further explore low temperature effects, titanium, zirconium and hafnium oxides were deposited on ultra-thin (~0.5 nm) SiO2 buffer layers and metastable states have been identified which give rise to large changes in the band alignments with respect to the Si substrate. Also discussed is the band edge electronic structure of 1) nanocrystalline Zr, Hf and complex oxide high-κ dielectrics, and 2) non-crystalline Zr and Hf silicates and Si oxynitride alloys. Three issues are highlighted: Jahn-Teller term-splittings that remove band edge d-state degeneracies in nanocrystalline films, intrinsic bonding defects in ZrO2 and HfO2, and chemical phase separation and crystallinity in Zr and Hf silicate and Si oxynitride alloys. Finally, photoemission spectroscopy has been used to characterize a candidate gate stack including electron affinities and work functions, valence band maxima, band bending in the Si and fields in the oxide layers. The band offsets are constructed and the deviation from the Schottky-Mott (or electron affinity) model at each interface is discussed in terms of interface bonding and the resultant charge transfer.